Well, a slice doesn't actually modify the audio file, you can still move the start and end points of the regions. So you can just 'Edit' one of the regions (say the one on the right), and drag the leftmost handle to the left so that it covers the other region.

1) Hotbox -> Edit the right region.
2) Drag the left circle handle all the way left.
3) Tap outside the region to commit the changes. You can always Undo from the menu if something goes wrong.